The present invention relates to investment casting, and more specifically to a spruing assembly for receiving investment material in the production of investment mold castings.
The castings of parts such as dental items or jewelry by the lost wax process is well known. The usual method is to provide an assembly which includes a base member and a casting ring associated therewith to form a hollow chamber to receive the investment material. The casting ring may be a metal ring with an asbestos or asbestos-substitute liner that permits the investment to expand slightly to result in larger castings. Alternatively, a plastic casting ring may be used which allows unimpeded setting and thermal expansion of the investment in all directions. The base member may be formed with or include a sprue base former containing a wax plug or sprue pin apertures for supporting sprue pins. The sprue pins, or a solid horizontal plastic feeder bar supported by sprue members, provide support for a wax pattern or patterns of the parts to be cast such as crowns, bridges, pontics, copings, inlays and other such items. The hollow chamber is then filled with high heat resistant investment material which is allowed to harden or set around the wax pattern and its supporting structure. After the investment has set, the base is removed and the ring is tapped or cut away to remove the investment assembly. The investment assembly is placed in a cold burn-out oven and heated to a temperature which causes the wax pattern to melt. When all of the wax pattern, feeder bar and sprue members melt, a cavity is left into which molten metal can be poured using a casting device. After the casting operation, the investment mold is broken apart from the cooled metal; and the individual parts, such as inlays, copings, crowns or bridges, can be cut off the cast sprue assembly and fitted onto a dental model.
One of the problems that arise with this process is that, since the wax patterns melt before the solid sprue or feeder bar melts, the molten wax cannot escape from the mold. Instead, the wax is driven into the investment and the carbon in the wax can contaminate the molten alloy during casting. The resulting cast parts are not suitable for use.
Another problem associated with this process is that, as the solid plastic feeder bar is heated, it expands. As it expands, it exerts pressures within the mold sufficiently powerful to crack and damage the mold. When molten alloy is cast into this mold, it provides enough impulse to crack the mold apart. This results in a potentially dangerous shower of molten alloy. Besides the loss of the cost of the alloy, the wax patterns must also be prepared anew which is a time-consuming and painstaking process.
Still another problem that arises with this process is that as the solid feeder bar or sprue member begins to melt, it will curl and crack or chip the enveloping investment. This may cause a distorted or incomplete casting.
A further problem associated with this process is that a solid sprue or feeder bar prevents the egress of volatized wax, steam and other vapors occurring within the investment mold that is being heated. As continued heating of the mold takes place but before the plastic of the feeder bar melts, the vapor pressures that accumulate may facture the mold with the same results as detailed above.
Accordingly, it is an object of the invention to provide a spruing assembly for producing high quality castings.
Another object of the invention is to provide an improved spruing assembly which eliminates the curling of the feeder bar and possible cracking of the investment mold used in the production of metal castings.
Still another object of the invention is to provide a new and improved feeder bar for a spruing assembly designed to provide faster elimination of the molten wax and vapors through its supporting hollow sprues.
A further object of the invention is to provide a novel feeder bar for a spruing assembly which does not exert any harmful stress on surrounding investment when the bar melts during burn out.